Method and apparatus for adjusting correlated color temperature

ABSTRACT

A color printing system includes a central processing unit for generating color image data, a color monitor for displaying color image data generated by the central processing unit and a color temperature sensing device for sensing color temperature of viewing light in which color images displayed on the monitor and color images printed by the printer are viewed. A first light source has a light output intensity controllable by the central processing unit. The central processing unit is adapted to control the intensity of the first light source in accordance with the color temperature sensed by the sensing device so as to match the color temperature sensed by the sensing device to a potential color temperature. Additionally, a second light source may be provided. The second light source has a different color temperature than the first light source and the central processor can adjust the intensity of the first and second light source based on relative color temperatures. The central processing unit is adapted to adjust the color temperature of either or both intensity of the first and second light sources.

This application is a division of U.S. patent application Ser. No.08/636,754, filed Apr. 19, 1996 (now U.S. Pat. No. 5,831,686 issued Nov.3, 1998), which is a division of U.S. patent application Ser. No.07/981,437, filed Nov. 25, 1992 (now U.S. Pat. No. 5,532,848, issuedJul. 2, 1996).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to adjusting correlated color temperaturein accordance with a detection of ambient or projected light. Inparticular, the invention relates to a method and apparatus forautomatically equalizing correlated color temperature of a displayedimage and correlated color temperature of viewing light, for example, byadjusting colors in the displayed image and/or by adjusting lightsources so as to vary correlated color temperature of viewing light.

2. Description of the Related Art

In color reproduction fields such as commercial printing andphotography, it is known that the correlated color temperature of theviewing light affects the way in which an observer perceives a colorimage. More particularly, an observer will perceive the same color imagedifferently when viewed under lights having different correlated colortemperatures. For example, a color image which looks normal when viewedin early morning daylight will look bluish and washed out when viewedunder overcast midday skies.

Correlated color temperature is characterized in color reproductionfields according to the temperature in degrees Kelvin (° K.) of a blackbody radiator which radiates the same color Light as the light inquestion. FIG. 1 is a chromaticity diagram in which Planckian locus (orhereinafter "white line") 1 gives the temperatures of whites from about1500° K. to about 10,000° K. The white color temperature of viewinglight depends on the color content of the viewing light as shown byline 1. Thus, the aforementioned early morning daylight has a whitecolor temperature of about 3,000° K. (hereinafter "D30") while overcastmidday skies has a white color temperature of about 10,000° K.(hereinafter "D100"). A color image viewed at D60 will have a relativelyreddish tone, whereas the same color image viewed at D100 will have arelatively bluish tone.

Because of these perceptual differences, conventional color reproductionpractice accepts 5,000° K. (hereinafter "D50") as a standard white colortemperature. In accordance with this convention, commercial colorreproduction facilities ordinarily evaluate color images for colorfidelity in a room whose light is controlled to a white colortemperature of D50.

Recently, however, low-cost high-quality color reproduction equipmenthas become available to individual users. Such users are not ordinarilyin a position to provide a room having ambient light controlled to D50.And, even if such rooms are available, the color image is not ordinarilydisplayed in a room whose ambient light is D50. Rather, such colorimages are more likely to be displayed in rooms not having a white colortemperature of D50 and may, for example, be used in an office buildingas part of a business presentation where the viewing light is fardifferent from D50.

Since the white color temperature affects the perception of color, ithas been proposed to modify the colors in a color image based on ameasurement of white color temperature of the viewing light. Forexample, "Color Equalization" by J. Schwartz, Journal Of Image ScienceAnd Technology, Vol. 36, No. 4, July/August, 1992, suggests to equalizea color image based on the white color temperature of viewing light byadjusting the amount of individual inks used during a printing processbased on the color temperature of the viewing light.

Heretofore, however, it has not been possible to measure the white colortemperature of viewing light and to automatically adjust a color imageproducing apparatus or viewing light in accordance with the detectedwhite color temperature. That is, conventionally, once a white colortemperature of viewing light had been determined, printing equipment,photography equipment, etc., had to be adjusted manually to match oraccommodate the viewing light. However, manual adjustment of equipmentor light sources is complicated, time consuming, and generally requirestrained technicians. Consequently, setting up controlled lightingconditions for viewing and displaying a color image throughout severalunrelated locations is very difficult and very expensive.

SUMMARY OF THE INVENTION

It is an object of the present invention to address the foregoingdifficulties.

The invention provides a digital color temperature sensor which providesa digital signal of white color temperature in response to a digitalrequest to provide the white color temperature. The color temperaturesensor device includes calibratable color sensors, and is mostpreferably fabricated on a single substrate or on a single VLSI chip.

According to this aspect of the invention, a color temperature measuringdevice for measuring the color temperature of light comprises asubstrate, a sensor fixed to the substrate for individually sensing theplural color components of light incident on the sensor and forproviding plural corresponding digital color component signalrepresentative of the color components, a memory fixed to the substratefor storing correction data for correcting the digital color componentsignals from the sensor, and a processor also fixed to the substrate andincluding a digital interface. The processor receives the plural digitalcolor component signals from the sensor, accesses the correction data inthe memory to correct the plural digital signals in accordance with thatcorrection data, derives a white color temperature from the correctedcolor component signals, and in response to a request to provide a whitecolor temperature, outputs the white color temperature on the digitalinterface. The digital interface may be provided by an input/output(I/O) interface which may also receive the request. The interface may beaddressable, thereby allowing the device to be connected to a serialline and monitor the serial line for requests that are specificallyaddressed to it. An alarm may also be provided to indicate that thesensed digital color component signals pertain to a light source whosechromaticity is outside the range of the white line.

In addition to the foregoing sensing mode, the device may also beoperable in a calibration mode, and in this case may be provided with aninternal light source such as an LED by which new correction data forthe sensors may be derived. In response to a command for entering thecalibration mode, the device sequences the light source through a seriesof different light levels, and rather than providing white colortemperature on the digital interface, the device provides the pluraldigital color component signals. Based on those signals, new correctiondata is derived, and the correction data is written, via the interface,back to the memory.

This brief summary of the invention is provided so that the nature ofthe invention may be understood quickly. A fuller understanding may beobtained by reference to the following detailed description of theinvention in connection with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chromaticity diagram showing the Planckian locus (orhereinafter "white line") in CIE space.

FIG. 2 is a perspective view of a color sensing device according to thepresent invention.

FIG. 3 is a functional block diagram of a color sensing device accordingto the invention.

FIG. 3A is a calibration device for calibrating the color sensingdevice.

FIG. 4 is a CIE chromaticity diagram showing isotemperature lines whichgive correlated color temperatures for illumanants which do not falldirectly on the white line of FIG. 1.

FIG. 5 is an elevational view of the physical arrangement of thecomponents shown in the FIG. 3 block diagram, and

FIG. 6 is a cross-sectional view along the line 6--6 in FIG. 5.

FIG. 7, comprised by FIGS. 7(a) and 7(b), is a flow diagram showingprocess steps by which the FIG. 3 embodiment interacts with requests ona serial line.

FIG. 8 is a block diagram showing an arrangement by which the colors ina color monitor and a color printer may be adjusted in accordance withthe color temperature of viewing light, and

FIG. 9 is a flow diagram showing the process steps for such anadjustment.

FIG. 10 is a block diagram view showing the arrangement of plural colortemperature sensors in different physical locations.

FIG. 11 is a block diagram view showing an arrangement by which viewinglight temperature and another color temperature may be matched to eachother, and

FIGS. 12 through 14 are flow diagrams showing methods for matching theviewing light temperature, in which FIG. 12 is a flow diagram foradjusting the viewing light temperature to match a desired colortemperature such as D65,

FIG. 13 is a flow diagram by which a monitor temperature is adjusted tothe viewing light temperature, and

FIG. 14 is a flow diagram by which viewing light temperature is matchedto that of a monitor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 is a perspective view of a correlated color temperature sensingdevice 10 according to the invention. The color sensing device includesa measuring head 11 comprised of an integrating sphere or a diffusinghemisphere by which ambient light such as light by which color imagesare viewed is collected and presented uniformly to a light sensor. Inthe case where the light sensing elements are integrated in a singlesubstrate as described below, measuring head 11 contains that substrate.Measuring head 11 is mounted on base 12 which provides support foroptional display 14 and alarm indicator 15. Display 14 displays acorrelated color temperature of the light striking measuring head 11; inFIG. 2 the numerals "65" are displayed indicating a correlated colortemperature of 6,500° K. or "D65"; and alarm indicator 15 visually warnswhen the light incident on measuring head 11 is so highly hued that itcannot be considered to be a white light and correspondingly does nothave a correlated color temperature. The operation of indicator 15 isdescribed in more detail below in connection with FIG. 4.

Serial cable 17 provides a digital I/O interface by which the sensorreceives requests and/or commands for operation and by which the sensorprovides a digital output of correlated color temperature. A suitableserial convention such as RS-232 may be used.

FIG. 3 is a block diagram showing the functional construction of acorrelated color temperature sensor according to the invention. Thecorrelated color temperature sensor includes three photosensors 21, 22and 23, each for sensing a separate color component of ambient light 24and for providing an analog signal representative thereof. In thepresent case, sensor 21 senses the red color component and provides ananalog signal therefor, sensor 22 senses the green color component oflight 24 and provides an analog signal therefor, and sensor 23 sensesthe blue color component of light 24 and provides an analog signaltherefor. In addition, there may be an optional sensor 21a for measuringthe blue contribution R1 to the red signal R. In this fashion, detectionaccuracy would improve. Each of the analog signals is converted byrespective analog to digital (A/D) converters 25, 26 and 27 and theconverted digital signals are led to multiplexer 29.

In response to channel information from microprocessor 30, multiplexer29 provides a selective one of the digital signals from A/D converter25, 26 or 27 via a data line to the microprocessor 30. Microprocessor 30may be implemented as a logical gate array, but more preferably it is aprogrammable microprocessor such as NEC V53. For each digital colorcomponent signal, microprocessor 30 accesses memory 31 for correctiondata to correct the digital signal for non-linearities, inconsistenciesand other errors in sensors 21, 22 and 23. Specifically, memory 31includes areas 31a, 31b and 31c for storing correction data for the redchannel, the green channel and the blue channel. The correction data maybe in the form of a simple bias and gain adjustment, but preferably thecorrection data is in the form of a look-up table by which the digitaldata from one of the A/D converters is used to look-up a corrected valuefor that data.

It is also possible to provide measuring head 11 with a temperaturesensor which is sampled by multiplexor 29 and an associated A/Dconverter to provide microprocessor 30 with the temperature of thesensors 21, 22 and 23 in the sensor head. In this case, the correctiondata also includes corrections based on temperature so as to allowmicroprocessor 30 to calculate temperature-compensated R, G, and B lightquantities.

After correcting each of the R, G and B components for ambient light 24,microprocessor 30 refers to a correlated color temperature table 31dstored in memory 31.

Correlated color temperature table 31d provides a correlated colortemperature based on the corrected R, G and B digital signals.Correlated color temperature refers to a situation in which the colorcontent of ambient light 24 is not exactly equal to any of the whitecolors indicated on line 1 of FIG. 1. The correlated color temperatureis defined as the temperature of the black body radiator whose perceivedcolor most closely resembles that of the given black body radiator atthe same brightness and under the same viewing conditions.

FIG. 4 shows isotemperature lines in CIE 1931 (x, y) space. Line 1 isthe same white line shown in FIG. 1. The additional lines which areapproximately perpendicular to line 1 are isotemperature lines. Thevalues stored in the correlated color temperature table 31d are suchthat colors falling on one of the isotemperature lines are followed backalong that line until it meets white line 1. The correlated colortemperature is considered to be the temperature at which white line 1 ismet. Thus, for example, ambient light whose color is such that its RGBvalues place it at the point indicated by reference numeral 2, then thecorrelated color temperature of the ambient light is 6,500° K. or D65.In this situation, even though the ambient light departs from a purewhite color, its departure is not so great as to consider it non-white,even though points above the white line 2 appear slightly greenish whilepoints below the white line 1 appear slightly pinkish.

On the other hand, light whose color components place it approximatelyoutside the areas indicated by dashed lines 3 are so hued that they canno longer be considered white. For light whose colors are outside thedashed region 3, such as light indicated by point 4, microprocessor 30uses correlated color temperature table 31d to generate a non-whiteindicator which is used to illuminate out of range indicator 15.

The correlated color temperature derived from correlated colortemperature table 31d is utilized to generate a signal to illuminateindicator 14. Thus, in the case of light whose color places it at point2, a signal "65" is generated corresponding to the 6,500° K. colortemperature of that light.

Reverting to FIG. 3, microprocessor 30 is preferably provided with aserial interface by which it may provide a digital signal representativeof the correlated color temperature not only to indicator 14 but alsoonto a serial line for communication to other digital equipment such asa personal computer. Interface 32 shown in FIG. 3 may be constructed ofa conventional universal asynchronous receiver/transmitter ("UART") bywhich serial requests received on serial line 17 may be processed and,if appropriate, a digital signal representative of the correlated colortemperature may be provided.

In addition to the color sensing mode described above, microprocessor 30may also be programmed to provide a calibration mode. In such acalibration mode, microprocessor 30 does not output correlated colortemperatures, but rather outputs uncorrected digital R, G and B signals.More particularly, in response to a command to enter a calibrate mode,which is illustrated schematically as a command from the serial line butwhich may also be a command formed from a simple push-button switchoperation, microprocessor 30 enters a calibration mode by whichuncorrected R, G and B values are output. The output values are comparedwith expected RGB output values. Thus, for example, those values arecompared with calibrated values which are expected by exposing thesensor to calibrated light. The expected values for each of the R, G andB components, together with the actual, uncorrected values for each ofthe R, G and B components are assembled into the R, G and B correctiontables 31a, 31b and 31c. The new correction data are provided tomicroprocessor 30, for example, over the serial interface, where theyare stored in memory 31.

In connection with the calibrate mode, the sensor may be provided with aself contained light emitting device such as LED 34. In response to acommand to enter the calibration mode, microprocessor 30 controls LED 34to illuminate at various pre-designated intensity levels. Since LEDshave stable color temperature values over their lifetimes, theuncorrected R, G and B output values may be compared readily to thosethat are expected from the pre-designated levels to which the LED isilluminated, thereby forming correction data for tables 31a, 31b and31c.

While LED 34 is illustrated as a single, whitish-output, LED, it is alsopossible to provide separate LEDs, such as a red, green and blue LED,whose combined light provides a whitish light. In this case, the LEDsshould be arranged so as to project light into measuring head 11 so asto allow the light to mix before illuminating the color sensors, therebyto minimize color crosstalk.

It is also possible to provide calibration LEDs in a separatecalibration device. For example, FIG. 3A illustrates a perspective viewwith. a cut-out section of the calibration device which. could be usedto calibrate the color temperature sensing device of the presentinvention. Calibration device 70 consists of hollow cylinder 71 havingopening 72 at one end. Opening 72 is large enough to allow the colortemperature sensing device to enter in the direction of Arrow A.

Cylinder 71 has approximately the same diameter opening 72 as themeasuring head 11 of the color temperature sensing device so thatcylinder 71 fits snugly over measuring head 11 in order to prevent straylight from entering the tube. To this end, the walls of bottom portion73 of cylinder 71 are painted black so as to form a light adsorbingsurface. The remaining interior 74 is coated with a white liningconsisting of any substance normally used for perfect white diffusers,such as polished opal glass, ceramics, and fluorinated polymer. At theopposing end of opening 72 of cylinder 71, there is disposed three lightemitting diodes (LEDs) 75, 76 and 77. Each LED is mounted for good heatdissipation on the top portion of cylinder 71.

LEDs 75, 76 and 77 are each of a different color and preferably, red,green and blue. In this manner, when LEDs 75, 76 and 77 emit lightsimultaneously, the combined colors mix to white. Any number of LEDs maybe used in any proportion to obtain a predetermined correlated colortemperature. For example, blue LEDs often emit less light than red LEDsso that in order to obtain white, blue LEDs should be present in alarger proportion. Moreover, the individual LEDs may be illuminatedindependently in order to obtain the same effect.

Power is supplied to calibration device 70 through cable 78 from plug79. Plug 79 is a feed through RS-232C connector and a Data TerminalReady Line may be used to branch off the required energy to calibratingdevice 70.

An optional LED 80 may be mounted to the extension of cylinder 11 toindicate to an operation that calibration device 11 is operational.

The calibration device is illustrated in FIG. 3A as a cylinder, butother configurations are possible, such as an integrating sphere havingan entrance aperture for receiving light from the LEDs and an exitaperture for emitted mixed LED illumination light. An internal bafflemay be provided to ensure light from the LEDs is shielded from directemission through the exit aperture.

To use, the calibration device 70 is placed over color temperaturesensor 10 which is operated in the calibration mode. The calibrationdevice 70 exposes the color sensors to whitish light and themicroprocessor 30 returns uncorrected RGB values as described above. Theuncorrected RGB values are compared with expected RGB values andcalibration tables are derived therefrom.

FIG. 5 is an elevational view of the structure of the sensor shown inthe FIG. 3 block diagram.

As shown in FIG. 5, the color temperature sensor is fabricated on asubstrate 40, which is shown as a dotted line in FIG. 3, in which areintegrated or fixed the color component sensors 21, 22 and 23, the A/Dconverters 25, 26 and 27, microprocessor 30, memory 31, and interface32. The device shown in FIG. 5 is also provided with an additional colorsensor 21a and corresponding A/D converter 25a which is designed tosense the blue contribution of the red signal and which may provide moreaccurate tristimulus R, G and B values. Substrate 40 may be anon-conductive substrate to which the individual components shown inFIG. 5 are mounted, but more preferably substrate 40 is a VLSI chip onwhich the components shown in FIG. 5 are fabricated in accordance withknown VLSI techniques. Not shown in FIG. 5 are connectors forinterconnecting between the individual elements on substrate 40 and forproviding external access to the color temperature sensor.

Sensors 21, 22 and 23 (and, if provided, sensor 21a) are notpre-sensitized to a particular color matching function. Rather, thosesensors are conventional photosensitive devices which are covered by afilter or other device for separating ambient light into red, green andblue tristimulus values. Thus, as shown in FIG. 6 which is across-section taken along line 6--6 of FIG. 5, red sensor 21 and greensensor 22 are each comprised by a conventional photosensing element 41covered by a filter 42 of appropriate color. Superimposed on each colorfilter 42 is a lenslet 44 which collects ambient light and inhibitslight scattering in the assembly. In this regard, further improvementsin sensitivity are obtained if areas away from the photosensing elementsare shielded by an opaque layer of material such as the layer indicatedillustratively at 45.

In operation, power from an unshown source is provided to the colortemperature sensing device, and the correlated color temperature sensingdevice is placed in position to collect.ambient light such as viewinglight for viewing a color printout. A user reads the correlated colortemperature of the viewing light from indicator 14 and verifies thatindicator 15 is not illuminated which would indicate that the viewinglight is too hued to be considered white. The user utilizes thecorrelated color temperature to ensure that color images are viewedunder the proper conditions. Thus, in one situation, a user may changethe color temperature of the viewing light, for example, by openingshades to outside windows so as to increase the color temperature or byilluminating incandescent bulbs so as to decrease the color temperature.Alternatively, a user may adjust the white point of a color monitor,which is the temperature of the color produced by a color monitor whenits red, green and blue guns are generating their maximum signals sothat it matches the color temperature of the illuminating ambient light.As yet another example, a user may enter the color temperature intocolor printing software which operates on the color temperature so as toequalize the colors printed by a color printer to the viewingconditions.

In the case where the color temperature sensor 10 is provided with aserial interface which allows access to other digital equipment, thatdigital equipment may use the color sensor in accordance with the flowdiagram illustrated in FIG. 7.

In step S701, microprocessor 30 initiates its line monitor loop. Theline monitor loop monitors the status of serial line 17 until a newstart character is detected on the serial line. Until a new startcharacter is detected on the serial line in step S702, microprocessor 30simply reinitiates its line monitoring operations (step S703) andremains in the line monitor loop until a new start character isdetected. When a new start character is detected on the serial line,flow advances to step S704 in which microprocessor 30 reads the addressof the recipient from the serial line. In more detail, several serialdevices are ordinarily connected to serial line 17. Each of the devices,including the color temperature sensing device 10, is accessed inaccordance with a unique address code. Thus, in step S704,microprocessor 30 reads the address code for the recipient from theserial line. If the address code does not correspond to the address ofcolor temperature sensor (step S705), then flow returns to step S703 inwhich the line monitor loop is reinitiated until a new start characteris again detected.

If in step S705 the microprocessor 30 determines that it has beenaddressed, then flow advances to step S706 in which the sender's addressis stored. The sender's address is used by microprocessor 30 ingeneration of a response. More particularly, microprocessor 30, whengenerating a serial response for the serial line, will preface thatresponse with the sender's address so that the response of thecorrelated color temperature device will be directed to the properrecipient.

Step S707 extracts the command which color temperature sensor 10 is toexecute. In more detail, microprocessor 30 may be programmed to provideresponses to different commands such as a command to provide thetemperature on the serial line, a command to enter the calibration mode,a command to receive and to store new correction data in the calibrationtables, or a command to reset to a new address. In step S707, thecommand is extracted.

In step S708, the command is inspected to determine if it is atemperature query. If the command is a temperature query, thenmicroprocessor 30 sends the correlated color temperature correspondingto the current ambient light 24 (step S709) and flow then returns tostep S703 where the line monitor loop is reinitiated.

If the command is not a temperature query, then step S710 determines thedistance of the correlated color temperature from the white line. If thecommand is a command to determine the distance, then the distance isdisplayed in step S711 and flow returns to step S703. If the distancequery has not been selected in step S710, but rather a command toinspect luminance is received (step S712), then in step S713microprocessor 30 sends luminance information out through the serialport.

If the command received is not a command for luminance data, but rathera request for RGB tristimulus values (step S714) then in step S715, theRGB values can be determined and output via the serial port.

If the command is not a temperature query command, then step S716inspects the command to determine if it is a command to enter thecalibration mode. If the command is a command to enter the calibrationmode, then microprocessor 30 enters the calibration mode wherebyuncorrected color components are transmitted on the serial line 17 (stepS718) and, if so provided, microprocessor 30 illuminates LEDs 34 (stepS717). As described above in connection with FIG. 3, LED 34 areilluminated to plural different pre-designated illumination levels, andthe uncorrected R, G and B components for those illumination levels aretransmitted via the serial line 17 to external calibration equipment.Flow thereupon returns to step S703 where the line monitor loop isreinitiated.

If the calibration mode has not been commanded, but rather a command toaccept new correction data is received (step S719), then in step S720microprocessor 30 stores a new correction data into R, G and Bcalibration tables 31a, 31b and 31c. As described above, thesecorrection data are utilized by microprocessor 30 to correct the digitaldata from A/D converters 25, 26 and 27 so as to compensate fornon-linearities, non-uniformities and other sources of errors in thedigital color components. Flow then returns to step S703 where the linemonitor loop is reinitiated.

If the command extracted in step S707 is not a command to store a newcalibration table, but rather is a command to accept a new deviceaddress (step S721) then flow advances to step S722 in whichmicroprocessor 30 stores the new address for device 10. Thereafter,microprocessor will only respond in step S704 to serial inquiries to thenew address. Flow then returns to step S703 where the line monitor loopis reinitiated.

The foregoing list of commands is representative only, and othercommands may be provided for by microprocessor 30. However, ifmicroprocessor 30 does not recognize the command extracted in step S707,then in step S723 it may be desirable to output an error signal in orderto notify the operator that the device is operational.

FIG. 8 is a constructional view of an arrangement by which computerizedfeedback is provided whereby the color output of a color monitor or thecolor images formed by a color printer are equalized properly forambient viewing conditions. In FIG. 8, host CPU 50, which may be aconventional personal computing system, is provided with a color monitor51, a keyboard 52 and a color printer 53. A color temperature sensingdevice 10 is connected to host CPU 50 via a serial interface 17 and isarranged to sense ambient viewing light for either or both of colormonitor 51 or color printer 53.

FIG. 9 shows process steps executed by host CPU 50 to equalize the coloroutput of monitor 51 or the colors printed by color printer 53 to theambient viewing light. In step S901, CPU 50 generates a temperaturerequest on serial line 17 which is addressed to correlated colortemperature device 10. Color temperature sensing device 10 responds tothe serial request as described above in connection with FIG. 7 and, viaserial interface 17, returns a digital representation of the colortemperature of the viewing light to CPU 50.

In step S902, CPU 50 determines if the monitor white point is equal tothe viewing light color temperature. If the monitor white point is notequal to the viewing temperature, then CPU 50 adjusts the monitor whitepoint (step S903) for example, by adjusting the gains of the red, greenand blue guns in color monitor 51.

In either case, flow then advances to step S904 in which CPU 50determines whether a print command has been received for printing acolor image on color printer 53. If a print command has not beenreceived, then flow returns to step S901 whereby CPU 50 constantlymonitors the temperature of the viewing light and equalizes the whitepoint of color monitor 51. On the other hand, if a print command hasbeen received, then flow advances to step S905 in which CPU 50 adjuststhe colors printed by color printer 53 so that they are equalized withthe viewing light's color temperature. Equalization of the kinddescribed in the aforementioned Schwartz article may be utilized ifdesired.

After equalization, flow returns to step S901 and the above operation isrepeated.

FIG. 10 shows an arrangement in which plural color temperature sensorsare arranged in different locations such as in different offices in aplace of business. Each of the color temperature monitors is providedwith a different serial address and each is connected to a serialinterface to network bus 50. By virtue of the foregoing arrangement, itis possible for a user in a first location such as in Office 1, whodesires to view a color image in a different location, such as during aconference or a meeting in Office 2, to read the correlated colortemperature in the viewing Light by causing a temperature request to beaddressed to the color temperature sensor in Office 2. Based on thecorrelated color temperature returned by the color temperature sensor inOffice 2, the user in Office 1 may modify the color printout on hisprinter so that the color image so generated will be equalized with theviewing conditions in Office 2.

In like manner, a user such as that in Office 2 who is not provided witha personal color printer may direct his color printer output to acentral location such as that shown in Office 3. In this instance, theOffice 2 user reads his correlated color temperature sensor beforequeuing a color print output, and the color print output is equalizedusing the Office 2 correlated color temperature, thereby providing forthe proper viewing conditions when the Office II user returns to hisoffice.

Office 3 is provided with its own color temperature sensor. This colortemperature sensor is utilized by the Office 3 color copier and thecolor facsimile unit in a manner similar to that illustrated in FIG. 9so as to equalize the color outputs of the color copier and the colorfacsimile to the ambient viewing light.

FIG. 11 depicts an arrangement for matching viewing light temperaturewith the temperature of another light, such as the white point of acolor monitor, or a standard day light simulator such as D50. In FIG.11, host CPU 50 is provided with a color monitor 51, a keyboard 52 and acolor printer 53. A color temperature sensing device 54 is provided tosense light 55 from color monitor 51 and to provide host CPU 50 with thewhite point temperature of monitor 51. A color temperature sensor 56 isprovided in an area away from color monitor 51 so as to sense viewinglight in the area. The viewing light is a combination of ambient light57, such as light from exterior windows, in combination with light 59which is from at least one controllable light source. In theconfiguration shown in FIG. 11, light 59 is from two light sources,namely incandescent source 60 or some other source having a relativelylow color temperature and fluorescent source 61 or some other sourcehaving a relatively high color temperature. The light intensity fromeach of sources 60 and 61 is independently controllable via intensitycontrol devices 62 and 64. Intensity controls 62 and 64 may befabricated from digitally controllable dimmer switches which areoperable under digital control from host CPU 50.

FIG. 12 is a flow diagram showing how the viewing light colortemperature and another light's temperature are matched. In the flowdiagram of FIG. 12, the viewing light is adjusted until the correlatedcolor temperature of the viewing light adequately simulates a desiredstandard illuminant such as D50.

In step S1201, CPU 50 reads the viewing light color temperature fromcolor temperature sensor 56 in accordance with the flow diagram depictedin FIG. 7. In step S1202, CPU 50 compares the viewing light temperatureto the desired light temperature such as D65. If in step S1203 theviewing light temperature is approximately the desired temperature, thenflow ends. On the other hand, if the viewing light temperature is lowerthan the desired color temperature (step S1204), then the viewing lightcolor temperature is raised by increasing the color temperature ofcontrollable light 59. In-the embodiment depicted here, this may beachieved either by decreasing the intensity of incandescent source 60(step S1205) or by increasing the intensity of fluorescent source 61(step S1206), or by any combination thereof. These adjustments may bemade by CPU 50 through incremental control of intensity controls 62 and64 whereby only incremental or step-wise changes are made in the colortemperature of light 59. Flow then returns to step S1201 in which thecolor temperature of the viewing light is again read to determine if ithas been brought to a level where it is equal to the temperature of thedesired color temperature.

If in step S1204, it is determined that the viewing temperature ishigher than that of the desired correlated color temperature, then theviewing light temperature must be lowered by, lowering the correlatedcolor temperature of adjustable light 59. In the embodiment depictedhere, this may be achieved either by increasing the intensity ofincandescent source 60 (step S1207) or by decreasing the intensity offluorescent source 61 (step S1208) or by an combination thereof. Asmentioned above, CPU 50 can effect these changes through digital controlof intensity controls 62 and 64, and preferably those changes are madeincrementally or stepwisely so as to effect only an incremental orstepwise change in the color temperature of light 59. Flow thereuponreturns to step S1201 so as to determine whether the desired colortemperature has been achieved.

FIG. 13 is a flow diagram showing another example for matching thecorrelated color temperature for the viewing light to another correlatedcolor temperature. In the flow depicted in FIG. 13, the colortemperature of the viewing light is not adjusted, but rather the whitepoint of monitor 51 is adjusted until it is the same as the colortemperature of the viewing light.

Thus, in step S1301, CPU 50 reads the color temperature of the viewinglight from color temperature sensor 56. In step S1302, CPU 50 determinesthe color temperature of light 55 from monitor 51 from color temperaturesensor 54. In step S1303 CPU 50 determines whether the colortemperatures are within a predetermined tolerance. If the colortemperatures are acceptable, then flow ends. On the other hand, if thecolor temperature of monitor 51 is low (step S1304), then flow advancesto step S1305 in which the white point of the color monitor isincreased, such as by increasing the gain of the blue gun or bydecreasing the gain of the red gun in the color monitor. The change maybe made incrementally or stepwisely thereby permitting the colortemperatures of the viewing light and the monitor to be matchediteratively. Flow thereupon returns to step S1301.

On the other hand, if the color temperature of monitor 51 is greaterthan the color temperature of the viewing light, then flow advances tostep S1306 in which the correlated color temperature of monitor 51 isdecreased, such as by decreasing the gain of the blue gun or increasingthe gain of the red gun of color monitor 51. Again, the change in colortemperature may be made incrementally so as to achieve correspondingincremental decreases in the color temperature of the monitor, wherebythe color temperature of the monitor and the viewing light are matchediteratively.

FIG. 14 is a further example of a method for matching the colortemperatures of the viewing light and that of another light. In theexample-of FIG. 14, the color temperature of the viewing light isadjusted so as to match the color temperature of that of monitor 51.

Steps S1401 through S1404 are identical to steps S1301 through S1304.

If in step S1404 CPU 50 determines that the correlated color temperatureof the monitor is lower than the correlated color temperature of theviewing light, then the color temperature of the viewing light isdecreased by decreasing the color temperature of light 59. In theexample herein, this is achieved by increasing the intensity ofincandescent source 60 or by decreasing the intensity of fluorescentsource 61, or any combination thereof. Any such adjustments are made byCPU 50 through intensity controls 62 and 64 and preferably madeincrementally so as to achieve iterative color temperature matching.Flow thereupon returns to step S1401.

On the other hand, if CPU 50 determines in step S1404 that the colortemperature of the monitor is higher than that of the viewing light,then flow advances to steps S1407 and S1408 in which the colortemperature of the viewing light is increased by increasing the colortemperature of light 59. In this example, the color temperature of light59 is increased by decreasing the intensity of incandescent light source60 (step S1407) or by increasing the intensity of fluorescent source 61(step S1408) or any combination thereof. CPU 50 affects theseadjustments incrementally via intensity controls 62 and 64 so as toachieve color matching iteratively. Flow then returns to step S1401.

In FIG. 13, only the color temperature of the monitor was adjusted andin FIG. 14 only the color temperature of the viewing light was adjusted,but it is to be understood that a combination of these effects may beutilized in matching the color temperature of the viewing light to thecolor temperature of the monitor. That is, it is possible to change thewhite point of the color 51 and also to change the color temperature oflight 59 in combination so as to achieve a match between the colortemperature of the viewing light and the color temperature of themonitor.

What is claimed is:
 1. A method for color printer comprising the stepsof:reading the color temperature of viewing light in a first locationfrom a second location; equalizing color output signals based on thecolor temperature read in said reading step; and printing a color imagein the first location based-on the equalized color data.
 2. A methodaccording to claim 1, wherein said reading step comprises the step ofaddressing a request for color temperature to an addressable colortemperature sensor connected to a digital interface.
 3. A methodaccording to claim 1, further comprising the step of viewing the colorimage printed in said printing step in the first location.